This application claims the priority of Japanese Patent Application No. 11-002416 filed on Jan. 8, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens for a high-density optical recording medium; and, in particular, to an objective lens for a high-density optical recording medium, which is used in an optical pickup apparatus for writing or reading information signals with respect to an optical recording medium such as an optical disk, a magneto-optical disk, an optical card, or the like for which high-density recording is demanded.
2. Description of the Prior Art
In recent years, CD (compact disk), MO (magneto-optical disk), DVD (digital video disk), and the like have come into wider use as an optical recording medium for recording audio information, visual information, data information for computers, and the like.
In optical pickup apparatus for writing or reading information signals with respect to these optical recording media, objective lenses having numerical apertures of 0.45, about 0.5 to 0.6, and 0.6 are in use for CD, MO, and DVD, respectively.
Also, since demands for smaller dimensions, lighter weight, and lower cost are strong, the optical pickup apparatus generally employ an aspherical single-element lens made of a synthetic resin material or glass material as the objective lens for satisfying these demands.
Meanwhile, along with dramatic increases in information recording capacities, there has recently been longing for the emergence of recording media whose density is higher even by a small amount. Therefore, efforts are always under way to improve the recording density of optical recording media.
Here, for improving the recording density of optical recording media, it is necessary to reduce the light-converging spot diameter caused by the objective lens. Letting xcex be the wavelength of a light source, and NA be the numerical aperture of the objective lens, the light-converging spot diameter is represented by the following expression:
kxc3x97xcex/NA
where k is a constant, whereby it is necessary to attain at least one of the shortening of the wavelength of the light source and the increasing of the NA of the objective lens in order to reduce the light-converging spot diameter.
The shortening of the wavelength of the light source has been improved by the shortening of the wavelength of semiconductor lasers and the development of SHG light sources.
On the other. hand, the increasing of the NA of the objective lens has conventionally been in progress by employing an aspherical single-element lens. This technique can achieve an NA of about 0.6 or less. For attaining a further higher NA, however, there has been a limit to the use of aspherical single-element lenses, since single-element lenses become harder to make. For example, the objective lens disclosed in Japanese Unexamined Patent Publication No. 10-123410 has a two-group, two element lens configuration in order to attain an NA of about 0.70.
However, the objective lens disclosed in the above-mentioned publication is harder to design and make as compared with the conventional single-element objective lenses, thus increasing labor and cost. Further, since the method of its evaluation is not easy, the labor and cost for inspecting the product would increase.
In view of the circumstances mentioned above, it is an object of the present invention to provide an objective lens for a high-density optical recording medium, which is easier to design, make, and inspect and can favorably correct aberration, while having a high NA of 0.75 or greater and a two-group, two-element lens configuration.
The objective lens for a high-density optical recording medium in accordance with the present invention is an objective lens for an optical pickup for converging a luminous flux onto an optical recording medium;
the objective lens comprising, successively from a light source side, a first lens made of a biconvex lens having at least one surface formed as an aspheric surface, and a second lens having a convex surface directed onto the light source side, the second lens satisfying the following conditional expression (1):
0.588 less than xcex22xe2x89xa60.676xe2x80x83xe2x80x83(1)
where xcex22 is the imaging magnification of the second lens.
Preferably, the second lens has a spheric surface on the light source side and a planar surface on the optical recording medium side.
Preferably, the first lens is constituted by a lens whose aberration is corrected by itself and satisfies the following conditional expressions (2) and (3):
NA1xe2x89xa70.45xe2x80x83xe2x80x83(2)
RMS1xe2x89xa60.07xcexxe2x80x83xe2x80x83(3)
where
NA1 is the numerical aperture of the first lens;
RMS1 is the RIMS of the wavefront aberration of the first lens; and
xcex is a reference wavelength.
Preferably, both surfaces of the first lens may be made aspheric as well.